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Chemical Equilibrium

How to use this simulation

The model reaction is A ⇌ B in a constant volume. K is [B]/[A] at equilibrium. Set K, the total concentration and the fraction of B you start with. The concentration readouts then move from that start toward equilibrium, and the graph shows the whole approach. The curves are an illustrative exponential relaxation, not a full integration of a school rate equation.

  1. Set a value of K and start with almost no B. [B] should rise and [A] should fall until the reaction quotient Q equals K.
  2. Start again with more B than the equilibrium mixture contains. The reverse reaction is faster at first, and the composition moves back to the same equilibrium.
  3. Increase K. The final mixture should contain a larger fraction of B. Decreasing K does the opposite.
  4. In advanced mode, change the total concentration and the approach rate. For A ⇌ B the equilibrium fractions depend on K, not on the total concentration. The approach rate only changes how quickly the readouts settle.

Key ideas

Dynamic equilibrium

At equilibrium the forward and reverse reactions are still happening. Their rates are equal, so the concentrations stop changing. A system at equilibrium is not a system in which the reaction has stopped.

K and Q

For A ⇌ B, K = [B]/[A] using equilibrium concentrations. The reaction quotient Q has the same form but uses the concentrations at the current moment. If Q < K, the forward reaction is faster and more B forms. If Q > K, the reverse reaction dominates until Q returns to K.

Le Chatelier’s idea, in this model

There is no temperature or pressure slider. Changing K stands in for a change, such as temperature, that alters the equilibrium constant. A catalyst would change the approach rate and leave K alone. Because there is one particle on each side, changing the total concentration does not shift the equilibrium fraction. That would be different for a reaction such as N₂ + 3H₂ ⇌ 2NH₃.

Reading the graph

Both curves are drawn for the whole approach. They level off at the equilibrium concentrations. The live readouts are the point the system has reached so far. Resetting a slider starts the approach again from the new initial composition.

Where this sits in the course

GCSE and IGCSE meet reversible reactions and the qualitative idea that a closed system can settle. A level and IB calculate K, compare Q with K, and use Le Chatelier’s principle. The concentration ratio here is the simplest Kc.

What the model leaves out

The mixture is ideal and the volume is fixed. The approach is a smooth first-order relaxation toward the required ratio. It is there to show the direction and the end point, not the order of a particular reaction.

More on the Chemistry simulations page, or the full SciSim library.